Mandibular incisor apicoectomy in a Canadian Beaver

Microstructurally driven self-sharpening mechanism in beaver incisor enamel facilitates their capacity to fell trees

Beavers (Castor) stand out among mammals for their unique capacity to fell trees using their large, ever-growing incisors. This routine consumption of resistant fodder induces prodigious wear in the lower incisors, despite this blunting effect the incisors maintain a remarkably sharp cutting edge. Notably, the enamel edges of their incisors show a highly complex two-part microstructure of which the biomechanical import is unknown. Here, using fracture analysis, nanoindentation, and wear testing on North American beaver (C. canadensis) incisors we test the microstructure's possible contribution to maintaining incisal sharpness. Although comparable in hardness, the inner enamel preferentially fails and readily wears at 2.5 times the rate of the outer enamel. The outer microstructure redirects all fractures in parallel, decreasing fracture coalescence. Conversely, the inner microstructure facilitates crack coalescence increasing the wear rate by isolating layers of enamel prisms that readily fragment. Together these two architectures form a microstructurally driven self-sharpening mechanism contained entirely within the thin enamel shell. Our results demonstrate that enamel microstructures exposed at the occlusal surface can markedly influence both enamel crest shape and surface texture in wearing dentitions. The methods introduced here open the door to exploring the biomechanical functionality and evolution of enamel microstructures throughout Mammalia. STATEMENT OF SIGNIFICANCE: Enamel microstructure varies significantly with the diversity of diets, bite forces, and tooth shapes exhibited by mammals. However, minimal micromechanical exploration of microstructures outside of humans, leaves our understanding of biomechanical functions in a nascent stage. Using biologically informed mechanical testing, we demonstrate that the complex two-part microstructure that comprises the cutting edge of beaver incisors facilitates self-sharpening of the enamel edge. This previously unrecognized mechanism provides critical maintenance to the shape of the incisal edge ensuring continued functionality despite extreme wear incurred during feeding. More broadly, we show how the architecture of prisms and the surrounding interprismatic matrix dictate the propagation of fractures through enamel fabrics and how the pairing of enamel fabrics can result in biologically advantageous functions.

[Extraction of diseased mandibular incisors in the guinea pig (Cavia porcellus) via ventral mandibular trepanation]

OBJECTIVE

This retrospective study describes the use of ventral mandibular trepanation for extraction of diseased mandibular incisors in guinea pigs and evaluates the clinical outcome.

MATERIAL AND METHODS

In 40 guinea pigs with abnormal feed intake, at least one mandibular incisor was extracted via ventral mandibular trepanation. The diagnosis of primary mandibular incisor disease was based on the findings of the clinical general examination and intraoral and radiographic examination under general anesthesia. Following the procedure radiographs were taken and the patients were monitored and cared for as inpatients until discharge. Regular re-examinations were undertaken in order to assess and treat the surgical sites.

RESULTS

A total of 42 altered mandibular incisors were extracted via ventral mandibular trepanation. Macrodonts (25/42) and periapical and alveolar changes (8/42) were the main reasons for extraction. Tooth residuals of unknown etiology (4/42) as well as such resulting from failed conventional extraction attempt using luxators (3/42) and traumatically loosened incisors (2/42) were also extracted. Tooth-associated jaw abscesses were found in 40 % of the guinea pigs. In addition, 40 % of the animals presented secondary elongation of the molars requiring occlusal equilibration. In all patients, it was possible to completely remove the altered incisors or residuals. On average, independent feed intake was observed 2 days after surgery and the animals were discharged. Complete healing of the surgical site was observed on average after 39 days (minimum 9 days, maximum 98 days). A total of 22 animals were evaluated 6 months following surgery and showed no further tooth-associated complaints.

CONCLUSION AND CLINICAL RELEVANCE

Ventral mandibular trepanation allows diseased mandibular incisors in guinea pigs to be removed entirely within one surgery and thus represents a curative procedure. Unlike the conventional extraction with luxators, it does not bear the risk of tooth fractures resulting in tooth fragments remaining in the alveolus.